A scanning tunneling microscope (STM) utilizing the principle of a so-called tunneling effect has been developed. That is, a metal probe having a sharpened tip is brought close to a sample. When a weak bias voltage is applied between them, electrons migrate through the gap between the probe and the sample. Either the probe or the sample is scanned in two dimensions, using piezoelectric devices, while maintaining constant the distance between the probe and the sample. In this way, the topography of the sample surface can be imaged at an atomic-scale resolution.
FIG. 3 schematically shows such a conventional scanning tunneling microscope. The body of this microscope is generally indicated by numeral 1, and comprises a probe 3, a scanner 4, an x piezoelectric device-driving circuit 5, a y piezoelectric device-driving circuit 6, a z piezoelectric device-driving circuit 7, a scan generator 8, a display unit 9, a bias circuit 10, a current-to-voltage converter amplifier 11, a logarithmic converter circuit 12, a comparator circuit 13, and an integrator circuit 14. Indicated by 2 is a sample. The scanner 4 comprises an x piezoelectric device 4X, a y piezoelectric device 4Y, and a z piezoelectric device 4Z.
In this scanning tunneling microscope 1, the z piezoelectric device 4Z is driven according to a height (i.e., a dimension taken along the z-axis) adjusting signal from the integrator circuit 14. The probe 3 is brought close to the sample 2 until the probe 3 is spaced an initial set distance d (nm) from the sample 2. Under this condition, the scan generator 8 produces x and y scanning signals to the x piezoelectric device 4X and the y piezoelectric device 4Y, respectively, to drive them. As a result, the probe 3 scans the surface of the sample 2 along the x- and y-axes.
At the same time, the bias circuit 10 applies a bias voltage between the sample 2 and the probe 3, thus inducing a tunneling current from the probe 3 to the sample 2. When the probe 3 scans the surface of the sample 2 along the x- and y-axes, the distance between the sample 2 and the probe 3 is varied due to topography of the surface of the sample 2. Accordingly, the distance between the sample 2 and the probe 3 is controlled so that the tunneling current is kept constant if the probe 3 scans along the x- and y-axes.
This control is described in further detail. The tunneling current from the probe 3 is converted into a voltage and amplified by the current-to-voltage converter amplifier 11 and sent to the logarithmic converter circuit 12. This logarithmic converter circuit 12 linearizes its input signal so that the output signal from the current-to-voltage converter amplifier 11 has a linear relation to the distance between the sample 2 and the probe 3. Then, the output signal from the logarithmic converter circuit 12 is fed to the comparator circuit 13, which in turn compares the output value from the logarithmic converter circuit 12 with a reference value corresponding to a preset value of the tunneling current. The comparator circuit 13 produces the difference between the output value from the logarithmic converter circuit 12 and the reference value to the integrator circuit 14, which integrates the output value from the comparator circuit 13. The z piezoelectric device 4Z is driven according to the output from the integrator circuit 14. Thus, the distance between the sample 2 and the probe 3 is controlled in such a way that the tunneling current is maintained constant.
The output from the integrator circuit 14 is also sent to the display unit 9, which also receives the output signal from the scan generator 8. In consequence, a topographic image of the sample 2 is displayed on the viewing screen of the display unit 9.
In the conventional scanning tunneling microscope 1, the sample can be scanned at a higher rate than the probe 2 that is lighter in weight. Therefore, it is common practice to adopt the probe scanning method, i.e., the probe 2 is attached to the front end of the scanner 4 and the surface of the sample 2 is scanned.
Atomic force microscopes (AFMs) for investigating a sample surface by measuring the physical force between the probe and the sample have also been developed. FIG. 4 schematically shows such a conventional atomic force microscope, which is generally indicated by numeral 15 and comprises a resilient cantilever 16, a probe 17 attached to the front end of the cantilever 16, a laser 18, a photodetector 19, a piezoelectric device-driving power supply 20, a scan generator 21, a central processing unit (CPU) 22, a display unit 23, a preamplifier 24, an error amplifier 25, and a mirror 26.
In this atomic force microscope 15, the laser 18 directs laser light at the top surface of the cantilever 16. The reflected light is guided to the photodetector 19 via the mirror 26. Under this condition, the probe 17 and the sample 2 are brought closer to each other until the distance between them becomes less than 1 nm. An atomic force that is either a repulsive or attractive force is exerted between an atom at the front end of the probe 17 and the surface atom layer of the sample 2. The probe 2 undergoes this force, deflecting the cantilever 16 upward or downward. This in turn changes the position on the photodetector 19 on which the laser light is incident. A signal representing the resulting change is produced from the photodetector 19 and sent to the piezoelectric device-driving power supply 20 via the preamplifier 24 and via the error amplifier 25. This power supply 20 provides a feedback control over the z (i.e., along the height) piezoelectric device 4Z so that the distance between the probe 17 and the sample 2 is kept constant (i.e., the atomic force is kept constant).
The output from the error amplifier 25 is associated with the output from the photodetector 19 and sent to the CPU 22. The output is displayed on the display unit 23. The probe 17 or the sample 2 is scanned in two dimensions while controlling the distance between the probe 17 and the sample 2. As a result, a topographic image of the surface of the sample 2, or a constant force image, is presented on the display unit 23. In this way, in the conventional atomic force microscope 15, the cantilever 16 held on its one side is used. Therefore, the sample 2 is generally attached to the front end of the scanner 4 and scanned.
If one tries to incorporate the functions of both scanning electron microscope 1 and atomic force microscope 15 into one scanning probe microscope, it follows that the probe scanning method and the sample scanning method are both used. This necessitates two scanners even if only the same scan is made.
The preparation of the two scanners doubles the cost. Furthermore, the two scanners must be adjusted separately. In this way, longer time and much labor are necessary to make adjustments.